1. Field of the Invention
The present invention relates to a semiconductor Hall element and a Hall sensor including a driving circuit for the semiconductor Hall element. In particular, the present invention relates to a Hall sensor capable of eliminating an offset voltage.
2. Description of the Related Art
At first the principle of detection of the presence of a magnetic field by a Hall element is described. When a magnetic field is applied perpendicularly to a current flowing through a substance, an electric field (Hall voltage) is generated in a direction perpendicular to both the current and the magnetic field. The principle of the magnetic detection by the Hall element is to acquire an intensity of the magnetic field based on a magnitude of the Hall voltage.
FIG. 3 is a diagram for illustrating the principle of an ideal Hall effect. On an ideal Hall element, a Hall voltage VH as an output from a voltmeter 3 is represented as:VH=μB(W/L)Vdd where W and L represent respectively a width and a length of a Hall element magnetism sensing portion 1, μ represents electron mobility, Vdd represents a voltage applied by a power supply 2 for supplying a current, and B represents an applied magnetic field. A coefficient proportional to the applied magnetic field B corresponds to the magnetic sensitivity, and hence a magnetic sensitivity Kh of this Hall element, is represented as:Kh=μ(W/L)Vdd 
On the other hand, in an actual Hall element, an output voltage is generated even when no magnetic field is applied. The voltage output under a magnetic field of zero is called offset voltage. It is considered that the offset voltage is generated when potential distribution inside the element becomes imbalanced by, for example, mechanical stress applied to the element from the outside or misalignment occurring in a manufacturing process. For an actual application, compensation for the offset voltage is necessary to be regarded as 0 volt.
The compensation of the offset voltage is generally carried out by the following method.
FIG. 4 is a circuit diagram for illustrating the principle of an offset cancellation circuit utilizing spinning current. A Hall element 10 has a symmetrical shape and includes four terminals T1, T2, T3, and T4 so that a control current is caused to flow between one pair of input terminals and an output voltage is obtained between the other pair of output terminals. When one pair of the terminals T1 and T2 of the Hall element serve as control current input terminals, the other pair of the terminals T3 and T4 serve as Hall voltage output terminals. In this case, when a voltage Vin is applied to the input terminals, an output voltage Vh+Vos is generated between the output terminals, where Vh represents a Hall voltage proportional to a magnetic field generated by the Hall element and Vos represents an offset voltage. Next, with the terminals T3 and T4 serving as the control current input terminals and the terminals T1 and T2 serving as the Hall voltage output terminals, the input voltage Vin is applied between the terminals T3 and T4 to generate a voltage −Vh+Vos between the output terminals. Reference symbols S1 to S4 denote sensor terminal switching means, and one of terminals N1 and N2 is chosen by a switching signal generator 11.
By subtracting one output voltage from the other which are obtained by the currents flowing in two directions described above, the offset voltage Vos may be cancelled to obtain an output voltage 2Vh proportional to the magnetic field (see, for example, Japanese Patent Application Laid-open No. Hei 06-186103).
However, the offset voltage may not completely be cancelled by this offset cancellation circuit. A description is now given for the reason.
The Hall element is represented as an equivalent circuit illustrated in FIG. 5. In other words, the Hall element may be represented as a bridge circuit in which the four terminals are connected via four resistors R1, R2, R3, and R4. Based on this model, a description is given of the cancellation of the offset voltage by carrying out the subtraction between the output voltages which are obtained by the currents flowing in the two directions as described above.
When the voltage Vin is applied between the one pair of terminals T1 and T2 of the Hall element, the following Hall voltage is output between the other pair of terminals T3 and T4.Vouta=(R2*R4−R1*R3)/(R1+R4)/(R2+R3)* VinOn the other hand, when the voltage Vin is applied between the terminals T3 and T4, the following Hall voltage is output between the terminals T1 and T2.Voutb=(R1*R3−R2*R4)/(R3+R4)/(R1+R2)*VinThen, the difference between the output voltages for the two directions is acquired as:Vouta−Voutb=(R1−R3)*(R2−R4)*(R2*R4−R1*R3)/(R1+R4)/(R2+R3)/(R3+R4)/(R1+R2)*VinThus, the offset voltage may be cancelled even when the respective resistors R1, R2, R3, and R4 of the equivalent circuit are different from each other, as long as R1=R3 or R2=R4. In this case, it is assumed that the respective resistances do not change even when the terminals to be applied with the voltage are changed. However, when this assumption is not satisfied, for example, even when R1=R3 for one direction but this relationship is not established for the other direction, the difference may not be made zero, and hence the offset may not be cancelled. A specific description is further given of one of reasons for the state in which the offset may not be cancelled by changing the application directions of the voltage.
The Hall element generally has such a structure that a peripheral portion of an N-type doped region to become the Hall element magnetism sensing portion is surrounded by a P-type impurity region for isolation. When a voltage is applied between the Hall current input terminals, a depletion layer expands at a boundary between the Hall element magnetism sensing portion and its peripheral portion. No Hall current flows in the depletion layer, and hence in a region of the expanding depletion layer, the Hall current is suppressed to increase the resistance. Further, the width of the depletion layer depends on the applied voltage. The resistances of the resistors R1, R2, R3, and R4 of the equivalent circuit illustrated in FIG. 5 are changed accordingly depending on the voltage application direction, and hence in some cases, the offset cancellation circuit may not cancel a magnetic offset.
There may be employed a method involving arranging depletion layer control electrodes around and above the element, and adjusting voltages applied to the respective electrodes, to thereby suppress the depletion layer from extending into the Hall element (see, for example, Japanese Patent Application Laid-open No. Hei 08-330646).
When the temperature in the Hall element 10 is not uniform, but has a distribution, the resistance in the Hall element 10 is not uniform, either, because the temperature is not uniform, resulting in locations low in the resistance and locations high in the resistance. On this occasion, the resistances of the resistors R1, R2, R3, and R4 have been changed by the temperature, and an attempt to cancel the offset by the spinning current thus fails.
Accordingly in a Hall sensor including a Hall element and an element serving as a heat source of a circuit for driving the Hall element, the offset voltage may not be eliminated by the spinning current method disclosed in Japanese Patent Application Laid-open No. 06-186103 since a temperature distribution is generated in the Hall element 10 due to the influence of heat generation.
Moreover, the resistances may be adjusted by the method disclosed in Japanese Patent Application Laid-open No. Hei 08-330646, but the method uses the plurality of the depletion layer control electrodes and requires a complex control circuit, and hence has such a problem that the chip size increases, which leads to an increase in cost.